1. Field of the Invention
This invention relates to a control apparatus for controlling a controlled variable of a controlled object by control inputs such that the controlled variable is caused to converge to its target value.
2. Description of the Related Art
Conventionally, as a control apparatus of this kind, the present assignee has already proposed a control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-23922. The control apparatus controls the rotational speed of a driven shaft as a controlled variable by inputting a control input to a clutch mechanism as a controlled object, and includes a controller for calculating the control input. This controller calculates the control input with a target filter-type two-degree-of-freedom sliding mode control algorithm, based on a controlled object model defining the relationship between the control input and the controlled variable. The control input is input to an actuator of the clutch mechanism, and the controlled variable is controlled such that it is caused to converge to its target value.
In the control apparatus configured as above, since the control input is calculated with the target filter-type two-degree-of-freedom sliding mode control algorithm, it is possible to separately change the rate and behavior of convergence of the controlled variable to the target value for adjustment, which makes it possible to ensure both high-level stability and accuracy of control.
When the conventional control apparatus described above is applied to a controlled object having extremal characteristics, described hereinafter, or a controlled object of a multi-input multi-output system (i.e. controlled object with a plurality of control inputs and a plurality of controlled variables), there is a fear that the stability and accuracy of control are degraded.
First, when the above conventional control apparatus is applied to a controlled object having characteristics that a controlled variable thereof takes an extremum value (maximum value or minimum value) in response to a change in a control input (hereinafter referred to as “the controlled object having extremal characteristics”), if a target value of the controlled variable is set to a value larger than the maximum value of the controlled variable or a value smaller than the minimum value of the same, the controlled variable cannot reach the target value, so that the control input is calculated such that the controlled variable is changed up to the maximum value or the minimum value. As a result, the controlled variable is controlled in a direction largely deviating from the target value. That is, the control system is made unstable, and the accuracy of control is largely degraded. Such a state is more liable to occur in the controlled object of a multi-input multi-output system than in the controlled object of a one-input one-output system.
Further, in general, in the controlled object of the multi-input multi-output system, a plurality of controlled variables are often in a mutually interacting relationship. The above conventional control apparatus, however, is configured such that a single controlled variable is controlled by a single control input, and hence interaction of one control input to another can cause an unstable behavior and a degraded rate of convergence of the controlled variable to the target value. To compensate for the inconveniences, if a lot of processes are to be executed for determining conditions and searching parameter maps, the size of the control program and the amount of mapping data used in the multi-input multi-output system become immense, which requires an increased capacity of a ROM for storing the data thereof. This results not only in increased manufacturing costs but also in increased computation load due to the increased size of the executed control program and the increased amount of data processed during the operation.